During the evolution of the synapsid reptiles into
the early mammals (a period of more than 150 million years), a number
of changes evolved in the endocrine and integumentary systems. The ancestral
hormone prolactin adapted to stimulate modified sweat glands into producing
milk. Hair covered the body (although this trait may have been present
in cynodonts). Endocrine control over normal physiological mechanisms
allowed for endothermy ("warm-bloodedness"). andditional changes
included the evolution of a variety of glands in the skin.
The hormone prolactin (which is shared among bony vertebrates including
fish) was used to stimulate modified sweat glands to roduce a fatty, protein
rich sweat called milk which nourishes their young. Hormones (such as
those of the thyroid) control ancestral reptilian mechanisms (such as
muscle contraction and the In the early mammals, parathyroid glands developed
from the dorsal portions of pharyngeal arches.
In the mammalian lineage, hair is thought to have evolved in the late
cynodonts, given that fossils suggest that the amount of adipose and vasculature
of the snout was similar to mammals which possess whiskers.
Milk is a form of modified apocrine sweat which is rich in proteins and
fats. In the platypus, milk simply oozes onto the surface of the abdomen
and the young lick it off of tufts of hair.
Enamel matrix proteins (EMPs), caseins, and proteins in saliva belong
to a family of genes known as secretory Ca-binding phosphoproteins, most
of which are located in the same cluster . EMPs include amenogenin, ameloblastin,
and enamelin. Milk may originally have served in the protection of eggs
from microbes. Casein is only known from mammals. As milk began to be
used as a food source in mammals, other genes such as a-lactalbumin (which
resulted from a duplication of lysozyme) were also expressed in milk
Sudoriferous and apocrine glands only exist in mammals. Sebaceous glands
secrete an oily sebum which functions as a conditioner for both skin and
hair. Sebaceous glands are similar in all mammals. Smegma and cerumen
are stale compacted sebum. Groups of sebaceous glands can form specialized
structures in certain groups such as harderian glands behind the eye,
preputial, and inguinal glands in rodents, abdominal glands in gerbils,
costovertebral glands of hamsters, inguinal glands of tamarins and marmosets,
labial glands of tarsiers, brachial glands of some lemurs, and the meibomian
glands on eyelid in all mammals. Mammalian sebaceous and apocrine glands
can secrete pheromones.
All amniotes have keratinized tips of their digits. Mammal claws are equivalent
to reptilian claws but are composed of a softer keratin.